Applicant's WO-A 90/11610, corresponding to U.S. application Ser. No. 07/496,460, describes an interface connection through a hole in an insulating part (referred to there as "electrically conductive feedthrough connection") which consists of a lead brazed into the hole and covered with active brazing material, the core of this lead having a coefficient of thermal expansion less than that of the insulating part.
The tightness of the interface connection, particularly if the insulating part is made of ceramic, glass, or single crystals, is essentially determined by the fact that after the active brazing material has cooled down, the insulating part must not be subjected to excessive tensile forces, since such forces would disrupt the structure of the material, thus making the joint between insulating part and lead leaky at this point.
Since, in the prior art arrangement, there is a certain choice of materials for the lead core which have a smaller coefficient of thermal expansion than the insulating part, and since it is difficult to make the coefficent of thermal expansion of active brazing materials smaller than that of the insulating part, the thickness of the covering of active brazing material and the diameter of the lead core or the ratio of the diameter of the lead core to that of lead as a function of the diameter of the hole must be carefully adapted to one another within very close limits; it may be necessary to implement such a small thickness of the covering of active brazing material that lead covered with active brazing material can only be produced with difficulty and at high cost.
Further prior art, which was partly published already a long time ago, only generally suggests to provide the insulating part with burnt-in metal areas--which is precisely what is to be avoided--or to solder the insulating part to the lead after deposition of metal on the insulating part (DE-C 946 074), or to braze a ceramic tubule into a stainless-steel tube of greater diameter by means of a titanium ring (GB-A 15 87 253). After the brazing, the titanium ring forms a brazing bead without the titanium having penetrated into the gap between the two tubes. In the case of the capacitor disclosed in CH-A 225 448, a similar brazing bead is present whose material has not penetrated into the gap between lead and insulating part, either.
To achieve vacuum tightness and avoid cracking in the cooled-down brazed joint, however, it is absolutely essential that the inside surfaces of the gap be brazed by the active brazing material. Consequently, the brazing material must move there, which can only be caused by capillary action. The latter is counterbalanced by the surface tension, however.
Since, furthermore, to avoid brittleness of the brazed joint, the proportion of active metal--this is at least one reactive element such as titanium, zirconium, beryllium, hafnium or tantalum--in the active brazing alloy can only be made so large that the insulating part will be just sufficiently wetted, the way shown by the abovedescribed prior art seemed impracticable to the inventors, because it did not result in the gap being wetted.
It is, therefore, an object of the invention to apply the available materials (insulating material; metals having a smaller coefficient of thermal expansion than the insulating part; active brazing materials) in a different and novel manner such that the covering of the lead with active brazing material can be dispensed with.